The invention relates to a method for operating an exhaust gas purification system connected to an internal combustion engine of a motor vehicle, comprising a selective catalytic reduction (SCR) catalyst for catalyzed reaction of nitrogen oxides contained in the exhaust gas of the internal combustion engine with ammonia, wherein a reducing agent containing ammonia is added to the exhaust gas upstream of the SCR catalyst at a predeterminable dosage rate.
DE 10 2008 036 88 A1 describes a process in which an exhaust system containing an SCR catalyst is supplied with aqueous urea solution as a reducing agent containing ammonia at a controlled adjustable dosage rate. The dosage rate is set as a function of various variables, so that either a nominal level of ammonia stored in the SCR catalyst predetermined by a computer model or a predetermined nominal efficiency for a nitrogen oxide conversion with ammonia stored in the SCR catalyst and/or fed into the SCR catalyst is at least approximately achieved. In this way, an effective reduction of nitrogen oxides from the exhaust gas of the corresponding motor vehicle internal combustion engine can be achieved.
The object of the invention is to provide a method which allows a further improved removal of nitrogen oxides from a motor vehicle's internal combustion engine exhaust gases.
In the inventive method for operating an exhaust gas purification system connected to a motor vehicle internal combustion engine comprising an SCR catalyst for the catalytic conversion of nitrogen oxides contained in the exhaust gas of an internal combustion engine with ammonia, an ammonia-containing reducing agent is added to the exhaust gas upstream of the SCR catalyst with a predetermined dosage rate, a pressure value correlated to an absolute pressure in the exhaust gas purification system on the inlet side of the SCR catalyst is determined, and the dosage rate is set at least as a function of the pressure value. The dosage rate is different from zero and is preferably set in order that a predetermined target value for a nitrogen oxide conversion or a reduction of nitrogen oxides contained in the exhaust gases is at least approximately achieved. The setting of the dosage rate is preferably controlled by a closed loop with feedback. However, a feed-forward control with an open loop is also possible. With the dosage rate according to the invention, which is dependent from the absolute pressure and in particular set in a controlled way, a further enhanced use of the conversion potential of the SCR catalyst is possible, and therefore a further enhanced reduction of nitrogen oxides from exhaust gases is made possible. The procedure according to the invention takes the findings of the inventors into account, i.e., that the absolute pressure influences particularly mass transfer processes, which, in turn, do also have a significant influence on the catalyzed nitrogen oxide conversion reaction. Among nitrogen oxides, hereinafter simply referred to as NOx, nitric oxide (NO) and nitrogen dioxide (NO2) are primarily considered.
It is particularly advantageous, according to the invention, if a NOx conversion of the SCR catalyst is determined and if it falls below a predetermined threshold for the determined NOx conversion, the absolute pressure in the exhaust gas purification system on the input side of the SCR catalyst is increased by increasing a flow resistance for exhaust gas flowing out of the SCR catalyst. The increase in the absolute pressure at a particular predetermined value on the input side of the SCR catalyst can be effected for example by actuating an exhaust gas retaining flap which is arranged in the flow direction behind the SCR catalyst in the exhaust gas purification system. By increasing the absolute pressure on the input side of the SCR catalyst, the pressure in the catalytic converter element itself is raised. As has been shown, it can thus positively affect the conversion of nitrogen oxides fed with the exhaust gas to the SCR catalyst with ammonia (NH3). The dosage rate can thus optionally be increased and an increased nitrogen oxide conversion can be achieved. Although the causal relationships can be considered as not completely clarified, an improved NOx conversion of the SCR catalyst can be considered as primarily caused by an increased storage capacity of NH3 as well as by a shift in the thermodynamic equilibrium of the conversion reaction.
Preferably, the absolute pressure of the SCR catalyst is increased in connection with a cold start or warm-up of the internal combustion engine. The heating of the SCR catalyst to temperatures at which this has a good NOx reduction activity occurs faster because due to the accumulation of exhaust gas and the correlated throttling effect, from the start hotter exhaust gases are emitted by the engine. If the SCR catalyst has reached a predetermined temperature of ca. 250° C. or a predetermined activity of ca. 70% NOx conversion, the increase of the absolute pressure may be reduced or completely reversed. An increase in absolute pressure has been found to be advantageous also after a warm-up of the internal combustion engine. At an operating temperature of the SCR catalyst at a temperature greater than about 250° C., in particular an increased load on the internal combustion engine with, for example, more than 70% of rated load may cause a relatively high NOx load of the SCR catalyst due to increased untreated NOx emissions. In such operating points a reduction of NOx emissions to specified limit values is often difficult. By increasing the absolute pressure on the input side of the SCR catalyst, an increase of its efficiency is made possible. As a result, even at operating points with high NOx concentrations in the inflowing exhaust gas in the SCR catalyst, high NOx reduction values and compliance with strict limits can be achieved.
In a further embodiment of the invention, it is provided that the increase in the absolute pressure is set as a function of operating variables of the internal combustion engine and/or of the SCR catalyst. Thus it is recognized that an increase in the absolute pressure on the input side of the SCR catalyst or in the SCR catalyst through the exhaust gas purification system has an influence on the internal combustion engine and affects its operation. Optionally there may be undesirable collateral consequences. Also, counterproductive effects on catalyst performance quantities may occur in conjunction with a positive impact on the NOx conversion. By increasing the absolute pressure in dependence on operating variables of the internal combustion engine and/or of the SCR catalyst, this cross-interference can be considered and adverse effects can be minimized or an optimum compromise be found with regard to counter-influenced operating parameters.
In a further embodiment of the invention, the increase in the absolute pressure is adjusted so that the NOx conversion of the SCR catalyst increases at least approximately by a predeterminable extent. For this purpose, it is preferable to resort to previously determined and stored characteristic curves or characteristic fields, which describe the pressure dependence of the NOx conversion of the SCR catalyst as a function of various operating variables. The operating variables may be one or more of the variables: exhaust gas flow rate, catalyst temperature, NO2— or NOx inlet concentration, NH3 slip and optionally other variables.
In a further embodiment of the invention, in parallel with the increase in the absolute pressure, a measure for influencing the exhaust gas temperature is taken on the inlet side of the SCR catalyst. In this way, at least approximately optimum operating conditions for the SCR catalyst with respect to its NOx conversion ability can be set. In particular, at low exhaust gas temperatures, for example 200° C. to 250° C., in parallel to an increase of the absolute pressure, measures to increase the exhaust gas temperature may be adopted. Conversely, for example, in case of exhaust gas temperatures higher than 450° C., measures may be adopted in parallel to an increase in absolute pressure to reduce the exhaust gas temperature. In order to influence the exhaust gas temperature, one or more operating parameters of the internal combustion engine may be modified, such as a change in the timing and/or quantities of fuel pre-, main and/or post-injection, exhaust gas recirculation rate, opening and/or closing times of the internal combustion engine intake and/or exhaust valves.
In a further embodiment of the invention, in order to increase the absolute pressure, a switching is provided of an exhaust gas flow path from a first flow direction in which the exhaust gas of the motor vehicle internal combustion engine before flowing through the SCR catalyst flows through a particulate reduction unit, to a second flow direction, in which exhaust gas from the motor vehicle internal combustion engine before flowing through the particulate reduction unit flows through the SCR catalyst. After switching the exhaust gas flow path, the particulate reduction unit is thus fluidly connected downstream of the SCR catalyst, while being upstream before the switchover. Thus, after switching, escaping exhaust gas from the SCR catalyst has to overcome the flow resistance of the particulate reduction unit prior to being discharged to the environment. The flow resistance of exhaust gas flowing out of the SCR catalyst and the absolute pressure upstream of the SCR catalyst are therefore increased and its NOx conversion capacity is also increased. The switching according to the invention of the exhaust gas flow path turns out to be advantageous, especially at low temperatures at which the SCR catalyst can achieve a NOx conversion rate of less than ca. 50%, especially without additional pressure increase because the particulate reduction unit as an upstream heat sink for the hot exhaust gas of the internal combustion engine is eliminated and the SCR catalyst thus receives hotter exhaust. This is advantageous in particular in case of a warm-up of the internal combustion engine.
When switching the exhaust gas flow path, it is also particularly advantageous when, in a further embodiment of the invention, exhaust gas in the second flow direction flows through the SCR catalyst and the particulate reduction unit in the opposite direction compared to the first direction of flow. By reversing the flow direction in the particles reduction unit a discharge of ash is allowed, which has accumulated in the first flow direction. If, as is preferred, a so-called ammonia barrier catalyst is provided downstream of the SCR catalyst, by switching the exhaust gas flow path in the second direction of flow, exhaust gas preferably flows through the same before flowing through the SCR catalyst. Due to the catalytic oxidation properties of the barrier catalyst it causes an increase of the ratio of NO to NO2 in the exhaust gas. This in turn allows an improvement in the catalytic yield of the SCR catalyst which is in a fluid downstream position in the second flow direction with respect to the barrier catalyst.
Advantageous embodiments of the invention are illustrated in the drawings and are described below. The features mentioned above and features still to be explained may be used not only in the respective feature combinations, but also in other combinations or alone, without departing from the scope of the present invention.